1. Technical Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor circuit in which semiconductor drivers of different withstand voltage are mixed and disposed within the same chip.
2. Prior Art
A semiconductor switch is used widely instead of a conventional relay and mechanical switch because it is small in size, capable of high speed operation and long in life. As indications representative of the performance of the semiconductor switch, there are withstand voltage, current capacity and on-resistance. By increasing the withstand voltage and current capacity, it is possible to increase treatable power which is a product of them. Further, by reducing the on-resistance, it is possible to decrease heat generation of the semiconductor switch and increase the current capacity under the same heat circumference. Thereby, the energy efficiency can be increased.
The technique for improvement of withstand voltage and current capacity of MOSFET used generally as a semiconductor switch is disclosed in “Application Technique of Power MOSFET” by Hiroshi Yamazaki, issued by Nikkan Industry Newspaper Co. pp. 55–57.
There is an intimate relationship between the withstand voltage and on-resistance of the MOSFET, and according to the above-mentioned literature page 55, there is the following relationship between on-resistance and withstand voltage of unit chip area of a power MOSFET:Rds (on)/unit chip areaBVds2.4–2.7 where Rds (on) is on-resistance between a drain and a source, and BVds is yield voltage between drain and source. That is, it is noted that when the withstand voltage is increased, the on-resistance increases rapidly in proportion to the 2.4 to 2.7th power of the withstand voltage. Namely, when the element is made so as to have withstand voltage more than the necessity, the on-resistance increases. From this fact, for the power MOSFET, design for optimum withstand voltage meeting with an intended use is important in order to decrease power loss and obtain high efficiency.
Further, in a lateral type power MOSFET, since yield voltage between the drain and the source is proportional to a distance between the drain and source, when the element is made so as to have withstand voltage more than the necessary voltage, a chip area increases. Therefore, the design for optimum withstand voltage meeting with an intended use is important also from a viewpoint of reduction of the chip area.